It is not new to reduce metal-oxygen compounds, preferably metal ores, in a molten bath and to supply the necessary energy to the smelt by carbonaceous fuels and oxygenous gases, and there are a number of protective rights and prior publications which deal with smelting reduction.
In steelmaking by the various air refining methods there were already efforts to reduce ores with carbon in a converter. The oxygen content of the blowing medium serves, among other things, to produce the necessary heat by oxidizing part of the carbon. German patent no. 605 975, from 1932, describes a method wherein the blowing medium and the carbon are separated from each other and supplied alternatingly to the smelt and, interestingly, the carbon was added in the form of a carbonaceous gas. This is also indicated by the claim with the following wording: "A method for making steel in converters or in air furnaces provided with tuyeres wherein ores are reduced in an iron sump and the carbon is added, carried by an oxygenous blowing medium, characterized in that air or oxygen-enriched air or pure oxygen and neutral gases or gases which release carbon themselves or have a reducing effect are alternatingly used as a blowing medium and carbon carrier."
An essential contribution to economical operation of the reduction of metal ores in a molten metal bath was made by afterburning the reaction gases, mainly CO and H.sub.2, in the gas space above the molten bath and recycling the resulting heat to the molten bath. The teachings on this afterburning of the reaction gases and the successful retransfer of the heat to the molten bath are described for the first time by the worldwide protected method for improving the thermal balance during steel finery, for instance U.S. Pat. No. 4,195,985. This patent print also states in col. 14, line 39, the use of iron ore instead of scrap as a coolant during steelmaking. A particularly advantageous form of this method and its further development to achieve higher afterburning rates and a special apparatus are set down in the internationally protected method and the apparatus for afterburning reaction gases, for instance in U.S. Pat. No. 5,052,918.
A well thought-out method for making iron/crude steel with a carbon content of 2 to 3% is described in German patent no. 33 18 005. In this process approx. 70 t crude steel are produced per hour in a melt-down vessel containing an iron smelt of approx. 120 t. The method is a combination system involving a melt-down reactor, a gas conditioning vessel and a shaft furnace for prereducing the ores. This method for making iron from ore is characterized in that the reaction gases emerging from the iron smelt are partly afterburned in the melt-down vessel whereby the resulting heat is largely transferred to the smelt and the reaction gases are cooled and reduced with reduction agents on the way to the ore reduction vessel. This process is characterized not only by the stated productivity but also by a comparatively small amount of recycle gas of 80,000 Nm.sup.3 /h with which 110 t iron ore are reduced to a degree of metalization of approx. 75%, and the gas then leaves the shaft furnace with a composition of approx CO 41 %, CO.sub.2 30%, H.sub.2 23%, H.sub.2 O 1%, N.sub.2 4%, to be subsequently used as a service gas, for example for heating purposes.
The hitherto described prior art clearly indicates steps which substantially contribute to an economical operation of a smelting reduction method. For example, whereas the basic considerations on the reduction of iron ores in steelmaking were set forth a relatively long time ago, the last-mentioned process describes in its examples the practical application of smelting reduction with production data and gas compositions and amounts. By contrast, many newly granted protective rights for smelting reduction contain only a row of known steps and no quantitative data on the quantity and materials balance of these processes. A random example of this is U.S. Pat. No. 4,985,068 whose main claim reads as follows: "A method for smelting reduction of iron oxide, comprising (a) feeding prereduced iron oxide into an enclosed smelter; (b) heating, melting and reducing said iron oxide to molten metal by combusting a surplus of natural gas with oxygen, carburizing the molten metal by dissolving dissociated carbon in the metal, and forming a reacted off-gas; (c) introducing hot air into the enclosed smelter above the molten bath and oxidizing a portion of the off-gas to produce a flue gas; (d) cleaning and cooling flue gas to a temperature of from about 800.degree. C. to 950.degree. C.; (e) contacting said iron oxide with said cleaned flue gas to perform the prereducing function; and (f) drawing off molten iron product."
At the European Ironmaking Conference in Glasgow in September 1991 the authors Cusack/Hardie/Burke presented an extensive report on the development of smelting reduction in their contribution "HIsmelt--Second Generation Direct Smelting," and this publication indicates a number of important process parameters and their mutual relations. It deals with the degree of prereduction of the ores as a function of the degree of afterburning of the reaction gases and the resulting coal required for ironmaking, as well as the stages of development of the smelting reduction methods known from industry and their essential characteristics. It states a simplified materials and thermal balance for the HIsmelt process, and mentions for the demonstration plant under construction a production capacity of 14 t pig iron per hour or 100,000 t per year.
Some common disadvantages are also indicated by the many prior publications on smelting reduction of metal ores and the combination of an ore prereduction stage with a melt-down vessel, and by the details known about the pilot plants and production facilities on this basis. The known production capacity, i.e. the metal production per unit of time, is relatively low. Limits probably result from the high energy turnovers in the melt-down reactor. It is also striking that, although there are differences in the amounts of gas to be removed from the process and their residual energy contents, considerable amounts of gas with relatively high thermal values must in any case be removed from the process. This even holds for methods wherein the partly afterburned gases from the melt-down vessel are utilized for prereducing ore with a relatively low degree of metalization. The economy of this processes remains contingent on the profits made in selling the surplus amounts of gas.